This invention generally relates to motor compressor units, and more particularly to lubrication systems therefor.
The utilization of hermetically and semi-hermetically sealed motor-compressor units has become increasingly prevalant in recent years, particularly in refrigeration applications where the motor-compressor unit is employed to compress refrigerant vapor. Conventionally, the motor-compressor unit includes a compressor, a motor such as an electric motor, and a shell enclosing both the compressor and the motor. The compressor defines one or more compression chambers, and a compression means, for example a reciprocating piston, is movably disposed within each compression chamber. The compressor also includes a rotatable crankshaft which is connected to both the motor and the compressor means; and the motor drives or rotates the compressor crankshaft, while rotation of the crankshaft moves the compression means within the compression chamber or chambers to compress vapor therewithin.
Often, the crankshafts of many motor-compressor units of the general type described above rotate at relatively high speeds, for example 3,500 rpm. As is obvious, at such high operating speeds, proper lubrication of the compressor elements such as the bearings or surfaces journaling the compressor crankshaft is very critical. Any lubrication problems, when operating at these high speeds, may result, for example, in bearing failure, eventually causing complete loss of the compressor. For this reason, the motor-compressor unit is usually provided with a supply of lubricant and a lubrication system to circulate the lubricant through the compressor.
Commonly, these lubrication systems includes a single stage lubricant pump. However, under many conditions it is preferred to provide a motor-compressor unit with a lubrication system having a two-stage lubricant pump. For example, if a compressor is designed to operate selectively at either half or full speed, a two-stage pump may be necessary to insure that the lubrication system provides adequate lubricant pressure at both of these speeds. In addition, a two-stage lubrication pump may be needed if the shell of the motor-compressor unit is filled with relatively hot, high pressure discharge vapors--as opposed to relatively cool, low pressure suction vapors, which is normally the case.
Specifically, as is known in the art, some vapors are naturally absorbed in the compressor lubricant and, as the lubricant is circulated through the compressor, the absorbed vapors tends to flash or boil out of the lubricant. The flashed vapors may collect within the passages of the compressor lubrication system, creating a pressure barrier to the flow of lubricant therethrough. Generally, the quantity of vapors which is absorbed in the lubricant and the associated pressure barrier to the flow of lubricant through the lubrication system tend to increase with the pressure and temperature of the vapors; and if the shell of the motor-compressor unit is filled with hot, high pressure discharge vapors, a single stage lubrication pump may not be sufficient to provide a proper flow of lubricant to the working parts of the compressor, necessitating a two-stage lubrication system. Moreover, even if a single stage lubricant pump provides adequate lubrication under the circumstances described above, a two-stage lubrication pump may be desirable to increase the flow of lubricant to the working elements of the compressor, thus increasing the cooling effect which the lubricant flow has on these working parts.
With conventional two-stage lubricant pumps, the compressor crankshaft extends downward into the lubricant supply and defines a multitude of radial and axial passages. As the crankshaft rotates, lubricant is drawn upward through a first stage, axially extending passage and then forced outward through a first radial passage, increasing the pressure of the lubricant. The lubricant is then conducted radially inward, through a passage defined by a thrust bearing located directly below the crankshaft, to a second stage, axially extending passage. Lubricant is conducted upward through the second axially extending passage and then again forced radially outward, further increasing the pressure of the lubricant. The lubricant is thence conducted via a plurality of grooves and bores defined by the crankshaft toward or to the various surfaces of the compressor requiring lubrication.
These prior art arrangements have a number of disadvantages or drawbacks. First, they require extending the compressor crankshaft into the lubricant. This, of course, increases the mass and cost of the crankshaft. Further, if the level of lubricant in the shell ever falls below the bottom of the crankshaft, the entire lubrication system is rendered ineffective. Second, with the prior art two-stage arrangements, the axially extending passage of one or the other or both of the two stages must be spaced from the rotational axis of the crankshaft, an arrangement which does not fully maximize the forces which the rotating shaft may apply to the lubricant. Third, the construction and design of the prior art systems have been somewhat complicated, often requiring, for example, the careful machining of one or more grooves or channels in the top surface of a thrust bearing which supports the compressor crankshaft.